The introduction of digital signal processing for electronic signal equalization following signal detection using digital coherent receivers has allowed the main fibre transmission impairments, like chromatic dispersion, CD, and polarization mode dispersion, PMD, to be compensated in the electronic domain rather than with optical devices. A digital signal processor comprising a digital equalizer arranged to perform as a two-dimensional match filter is able to compensate for both CD and PMD, to recover a signal following propagation across an optical link. The digital equalizer operates to automatically tend to take the form of the inverse of the channel linear transfer function of the received signal. As reported by Hauske et al, “Optical performance monitoring in digital coherent receivers”, J. Lightwave Technol., vol. 27, no. 16, 2009, pages 3623-3631, the concept of optical performance monitoring by estimating deterministic linear channel distortions like residual CD, PMD, and polarization dependent loss, PDL, by analyzing the finite impulse response of the digital equalizer has been shown in theory and by simulation. However, Hauske et al also report that although the theory of the linear optical channel model enables all the desired optical parameters to be monitored, there remain problems with the practical implementation of the estimation algorithms that are used.